For TV tuners included in television receivers tuner circuits are often used having a loop through function for passing the HF input signal, for example an antenna signal, to a HF output, so that the input signal can be used also for a second television receiver or for a video recorder. To the HF output also the output signal of a TV modulator may be coupled, to provide for the subsequent appliance the baseband signal of the television program, as selected by the television receiver. A tuner circuit comprising an active amplifier for the loop through function is known for example from EP 769850.
The loop through function should operate also, when the television receiver is switched-off, to allow the operation of the subsequent television receiver or the video recorder also in the case, when the first television receiver is switched-off. Due to energy saving, it is advantageous that the power consumption of the loop through function is zero or as small as possible, when the tuner circuit and the respective television set are switched-off.
A loop through function not requiring any power when the tuner is switched-off is possible when using a MOSFET circuit including a depletion type MOSFET. This type of MOSFET is switched-through when the supply voltages at the MOSFET are zero. The depletion type MOSFET is blocking, if the gate-source voltage has a sufficiently negative value.
To provide a low insertion loss when the MOSFET is switched-through and a sufficiently high isolation when the MOSFET is blocking over a bandwidth of about 50 MHz to 800 MHz, as required for television tuners, specially designed MOSFETs have to be used, for the example the RF switch BF1107 provided by Philips Semiconductors. Technical details and applications for this RF switch can be found in the Philips RF Manual, 3rd edition, Appendix D, July 2003, pages 40-45, document number 432225206385.